The present invention relates to the field of optical disc recording and playback apparatus. More specifically, in one embodiment the invention provides a method and apparatus for improving the quality of recorded data on an optical disc and, more particularly, a method and apparatus for varying the amplitude and timing of an input write signal prior to its being applied to a write beam modulator in an optical recording system.
Optical discs and methods for recording and erasing data on optical discs are well known. For example, Feyrer et al., U.S. Pat. No. 4,719,615, discloses an optical recording media and method of recording thereon and is incorporated herein by reference for all purposes. In Feyrer et al., a laser recording beam is directed at a recording media which includes a material which expands upon heating. When the recording beam is directed at the material, the expansion material is retained in an expanded state, creating a data bump or mark.
An improved recording media and method for recording thereon is disclosed in U.S. application Ser. No. 294,723, assigned to the assignee of the present invention and incorporated herein by reference for all purposes. Copending application Ser. No. 294,723 discloses, among other things, a method and apparatus for recording and erasing optical discs which requires only a single wavelength laser beam for erasing, writing, and reading from the media.
In some prior optical recording systems the write beam is controlled by a modulating signal which includes a series of pulses which are 0 or 3 volts. In most compact disc applications, the pulses range in time from 3T to 11T where "T" is a clock period used in the recording device (typically 231 nanoseconds).
In standard compact disc recorders and players, 8,14 code (otherwise referred to as eight-to-fourteen or EFM) is utilized. Eight-bit binary numbers in EFM are represented by 14-bit channel codes. Only 258 distinct codes of 16K possible combinations are used to ensure that the run length is not less than 3T or greater than 11T, to allow for packing bits, and the like. Decoding is typically performed with a lookup table. Channel coding for optical discs and other recording media is described in Watkinson, The Art of Digital Audio. Focal Press, Chapters 6 and 8 (1988), which is incorporated herein by reference for all purposes.
A method/apparatus for recording digital signals is also disclosed in Tanaka et al., U.S. Pat. No. 4,425,637. The leading edge of a digital binary signal used to write on an optical disc is advanced to decrease the effective spot size of a recorded mark. To increase effective spot size, the digital read signal time period is increased by delaying trailing edges of the digital signal. All recorded marks provided by Tanaka et al. are delayed or advanced. No method or apparatus for modulating record beam amplitude, or for accounting for differences in record signal length is shown or suggested.
While prior art recording systems for optical discs have met with some success, certain problems are created, particularly when the marks recorded on the disc reach a very small size. When very small mark sizes are used there is frequently insufficient correspondence between a write beam modulating signal and the recorded bumps or marks. For example it has been observed by the inventors herein that with prior art recording systems a mark formed by application of a laser beam for a time period of 3T may actually be substantially longer than 3T on the disc. Also, the leading and trailing edges of such marks are often ill-defined. Further, the center of a mark may not be flat, but may have higher and lower points therein. The recording beam spot size and the characteristic media response dimensions, therefore, have become limiting factors in such recording systems. Further, even when recorded marks are "perfect" an electrical playback signal produced from the marks may be flawed, i.e., the signal length produced during playback of the mark may not correspond well with the intended mark length due to the finite size of a playback light beam.
Accordingly, it is seen that an improved method and apparatus for modulating a write beam in an optical data recording system is desired.